The present invention broadly relates to methods for estimating the load on an engine exhaust gas particulate filter resulting from the accumulation of particulate matter, and deals more particularly with an improved method for determining the load that includes an estimation of the ash content in the filter.
Particulate filters are used in the exhaust systems of internal combustion engines, especially diesel engines, (diesel particulate filters or DPF) to trap and remove particulate matter which is primarily formed of carbon based material. As the engine exhaust passes through the DPF, the particulates are trapped in the filter and accumulate over time. This leads to an increase in the resistance of the exhaust gas flow through the DPF, and therefore, to an increase in the backpressure on the engine. This increase in backpressure has an adverse effect on engine operation, and especially on fuel consumption. In order to reduce backpressure to acceptable levels, the DPF is periodically regenerated by burning off the accumulated particulates, most of which are combustible.
The DPF should be regenerated sparingly, however, for a number of reasons. First, the filtering efficiency of the DPF increases with increased particulate loading. Second, in order to achieve regeneration, the exhaust gas temperature must be elevated to above that achieved during normal engine operation, consequently regeneration results in higher fuel consumption. Third, regeneration is most effective when the particulate loading is sufficiently high and homogenous throughout the filter.
There are a number of conventional methods that may be used to increase the exhaust gas temperature to the requisite level (e.g. above 450xc2x0 C.) in order to initiate a regeneration event. Regardless of the method used to increase exhaust gas temperature, it is necessary to estimate the load on the DPF so that regeneration events are initiated at optimal intervals. The DPF load may be inferred from reference values that depend on engine operating conditions, or it can be measured more directly by sensing the exhaust pressure upstream and downstream of the DPF and solving an equation describing the relationship between the mass flow through the DPF and the sensed pressures. The effective restriction used in this equation establishes the dependence on the load accumulated in the DPF. By first solving the equation for the effective restriction and then solving the equation describing the effective restriction for the load, the load of the DPF can be estimated.
Known techniques for estimating DPF load do not take into consideration the accumulation of inert particulate matter in the DPF, such as ash. Ash adds to the backpressure experienced by the engine and to the pressure drop across the DPF, although it does not endanger the DPF during regeneration since it does not contribute to the exothermic reaction that occurs during the regeneration process. It would be highly desirable to distinguish between the amount of ash accumulated in the DPF, and the load represented by the carbon particulates accumulated in the DPF. The present invention is directed towards satisfying this need.
The present invention provides an improved method for determining the load on a (DPF) which includes estimating the amount of the load attributable to inert material, such as ash.
According to one aspect of the invention, the amount of inert material accumulated in the DPF is determined by detecting the end of a DPF regeneration event and then determining the existing DPF load which is assumed to be produced by inert particulates. The end point of the regeneration event may be determined by sensing the temperature of the exhaust gas flowing through the filter or by determining the rate of change of the DPF load. The remaining load in the DPF representing inert material may be determined by sensing the pressure drop across the DPF, immediately following the end of the regeneration event. Preferably, the backpressure load in the DPF is measured over a number of regeneration events, and the inert particulate load is taken as the smallest load value determined after all of these regeneration events.
According to another aspect of the invention, a method is provided to estimate the amount of inert particulate load in a DPF, comprising determining the total DPF load over a pre-selected time or distance interval and selecting the smallest load value as being that attributable to the inert particulate load. The method is preferably formed by storing only two time/load or distance/load pairs in order to avoid the need for storing the entire set of DPF load data over the full pre-selected time or distance interval.
In accordance with the preferred embodiment, of the invention, the DPF load is estimated using equations describing laminar or turbulent flows in pipes that represent the relationship between the mass flow through the DPF and the pressure drop across it.
Accordingly, it is a primary object of the present invention to provide an improved method for determining the load on a DPF.
Another object of the invention is to provide a method as described above which estimates that portion of the DPF load that is created by inert particulates in the DPF, such as ash.
A further object of the invention is to provide a method as described above that determines the DPF load by sensing the pressure drop across the DPF and then solving equations that describe the laminar or turbulent flow through the DPF.
These, and further objects and advantages of the invention will be made clear or will become apparent during the course of the following description of a preferred embodiment of the invention.